This invention relates to catalyst recycle, and more particularly to the conversion and recycle of bromide-containing constituents of catalyst compositions used to conduct chemical reactions.
The use as catalysts of various compositions comprising transition metals is known. Among the catalyst constituents in common use are compounds of the Groups 8, 9, and 10 metals, including those having atomic numbers of at least 44 (i.e., ruthenium, rhodium, palladium, osmium, iridium and platinum), hereinafter "heavy transition metals". Compounds of other metals, such as those of Group 8, 9, and 10 with atomic numbers less than 44, and including cobalt, lead, manganese, cerium, titanium, and copper are also useful as constituents of said catalyst compositions.
Bromide sources are also frequently present in various catalyst compositions. These include alkali metal and alkaline earth metal bromides, and onium salts, including trialkylamine hydrobromides and tetraalkylammonium, tetraalkylphosphonium, hexaalkylguanidinium, and sulphonium bromides.
The reactions in which such compounds serve a catalytic function include some which involve organic compounds as reactants, products or both. An illustration is the catalytic carbonylation of hydroxyaromatic compounds such as phenol with carbon monoxide and oxygen to yield diaryl carbonates such as diphenyl carbonate. This reaction will sometimes be designated "carbonylation" hereinafter.
In a typical carbonylation reaction, phenol is combined with a compound of a heavy transition metal, most often palladium, and other catalytic species which may include organic and inorganic co-catalysts and at least one bromide source. One or more other metal compounds, most often of lead, may be used as inorganic cocatalysts, and the use in combination therewith of bromide sources which may include alkali metal or alkaline earth metal bromides, tetraalkylammonium bromides, or hexaalkylguanidinium bromides is frequently advantageous. The resulting mixture is pressurized with carbon monoxide and oxygen to yield a product mixture containing diphenyl carbonate, unreacted phenol and by-products which include palladium in elemental and/or combined form and compounds of other metals present in the catalyst composition.
In U.S. Pat. No. 5,981,788, there is described a method of recovering and recycling carbonylation catalyst constituents which include heavy transition metals such as palladium, other metals including those of the Groups 8, 9, and 10 with atomic numbers less than 44 such as cobalt, and bromide sources such as tetraalkylammonium and hexaalkylguanidinium bromides. The recovery of the bromide source comprises simply its extraction with water, optionally following a preconcentration to lower the phenol concentration. This is practical when the other metal is cobalt, since there is essentially no chemical interaction in the presence of cobalt between hydroxyaromatic compounds such as phenol and bromide ions.
However, many of the numerous other metals employed in such catalyst compositions are not as suitable as cobalt in maintaining the presence of ionic bromide. This is particularly true of lead, which, when present, promotes conversion of the ionic bromide, typically an amount in the range of about 30-95% by weight thereof, to covalently bound bromine in the form of organic bromine compounds, predominantly bromophenols such as 2- and 4-bromophenol. Other metals can produce the similar covalently bound bromine compounds.
Conversion of ionic bromide to covalent bromine makes recovery of the cationic portion of the bromide source difficult. This may be the result of association of the cation with organic anions such as phenate. In addition to this difficulty of recovery, most ionic molecules in which the anion is a non-halogen such as phenate are not appreciably active as catalyst constituents.
It is of interest, therefore, to provide a method for recovery of bromide sources and their conversion or reconversion to compounds capable of supplying bromide ion to a catalyst composition.